The present invention relates to a welding and cutting device for film-wrapped packaging.
Various types of packaging machines exist on the market which, using the flow wrap or flow pack process, wrap the objects to be packaged in a thin film. The film which is wrapped around the products to be packaged is joined in the longitudinal direction by welding together the overlapping edges of the film which, together with the products wrapped in this way, is conveyed along a horizontal plane toward a transverse welding and cutting unit.
The known welding unit basically consists of two horizontal shafts, each shaft being fitted with one or more plates which project radially and in opposite directions and carry means for welding and means for cutting the film. In the following text reference will be made to double plates but, needless to say, the invention may also be applied using a greater number of radially projecting plates.
The transverse welding and cutting of the packaging film takes place each time the free ends of two plates meet above and below the film to be welded and cut during the rotary motion of the supporting shafts.
Depending on the product to be packaged, wrapping films with different technical characteristics are used. Thus, the temperature at which the films can be welded, the size of the films and their tear strength, in addition to the thickness of the films used, which can range for example between 20 and 100.mu., all vary in each individual case. In addition, the temperature to which the plates are heated during closure and welding of the packages also needs to be varied, these temperatures ranging approximately from 50.degree. to 250.degree. C. The temperatures selected depend on the contact time between the plates and the film, i.e. on the production rate.
In order to produce perfect packages and reduce rejects and waste to a minimum, it is vital that the rotating shafts and the plates for welding and cutting the film are mounted precisely parallel with one another, even in cases of inevitable thermal expansion, which occurs as a result of the different heating temperatures.
Moreover, an extremely accurate distance needs to be set between the welding and cutting plates, this distance depending on the type of film, the film thickness and the thermal expansion of the mechanical devices. The setting of this distance also depends on the cutting position which needs to be generated between the blade and the counterblade of the welding plates.
Therefore, in known machines it was hitherto vital, before the start of production, to carry out laborious manual setting operations on the basis of the film used, accurately positioning the ends of the rotary shafts which support the welding and cutting plates. In addition, it was necessary to perform laborious manual adjustments on screws and springs, in order to set the desired coupling pressure and distance between the welding plates, so as to avoid excessive cuts in the film in its transverse welding zone.
It is obvious that these very delicate operations, which are needed to set the coupling distance between the welding and cutting plates and to set the pressure exerted on the film by the welding elements and the cutting blades, are fundamental to the efficient operation of the machine, which is designed for continuous and high-speed production.
In the case of machines of the prior art, these setting operations had hitherto to be performed by an experienced operator each time there was a change in production requiring a different type of film or varying the production rate, at which point the operator would have to set the temperature of the rotary welding plates appropriately. Moreover, it was necessary to set the coupling distance between the welding and cutting plates by hand, which operations, on account of the need to carry out numerous trial attempts, led to lengthy and undesired machine stoppages.
It should be emphasized that in the known machines, for reasons of safety and as a result of constructional characteristics, all setting operations had to be carried out while the machine was stopped, a fact which considerably affected the productivity of the whole production plant.